THE AMERICAN BIOLOGY TEACHER HYDROGEN BONDS 343
Using the Boiling Point of Water
to Learn about Hydrogen Bonds
b s t r A c t
The boiling point of water is 300°C higher than expected because of hydrogen
Water; hydrogen bonds; boiling point.
The boiling point of water can be used to illustrate the importance of
hydrogen bonds in living systems. The boiling point of a compound
is the temperature at which a liquid becomes
a gas. All other things being equal, the boiling
point should be approximately proportional
to molecular weight, since molecules are held
together by weak forces that are roughly pro-
portional to mass. When the temperature is
high enough to separate the individual mol-
ecules from each other, they boil, or become
I give students the following chart:
Molecular Weight Substance Boiling Point
I then fill in the boiling points as follows. I round to the nearest
whole number, but here I give the actual numbers for reference.
Absolute zero is –273°C, or 0°Kelvin.
The value for CO
is the sublimation point,
when dry ice, a solid, becomes a gas.
I then ask students to predict the boiling
point of H
O. They admit that it should be
around –200°C. Of course, we all know that
water boils at +100°C. The hydrogen bonds
between the water molecules raise its boiling
point by approximately 300°C, thereby causing
water to be a liquid at room temperature and making oceans, rivers,
lakes, and, indeed, life possible.
This year, my students commented that they knew everything I
was telling them. They knew that N
, and CO
are all gases at room
temperature. But they had never quite thought about it this way.
SUSAN OFFNER is a biology teacher at Lexington High School, Lexington,
MA 02421; e-mail: firstname.lastname@example.org.
The American Biology Teacher, Vol. 74, No. 5, page 343. ISSN 0002-7685, electronic ISSN 1938-4211. ©2012 by National Association of Biology Teachers. All rights reserved.
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The boiling point of
a compound is the
temperature at which
a liquid becomes a gas.